The present invention relates to an improvement of a pulse-width modulation (PWM) circuit suitable for controlling a disk motor of a DAD (digital audio disk) reproduction circuit of a CD (optical compact disk) type.
A PCM (pulse code modulation) digital recording/reproduction system has been recently adopted in order to achieve a higher fidelity in acoustic equipment. This is because a digital audio technique has been established which provides better audio characteristics than the conventional analog recording/reproduction systems without dependency on the characteristics of a recording medium.
Among the digital audio systems, there is a system called a DAD system which records signals on and reproduces them from a recording disk. Various recording/reproduction systems for the DAD system have been proposed such as optical, electrostatic and mechanical systems. Whichever one of these recording/reproduction systems is used, the data reproduction apparatus must satisfy various control functions and high-performance requirements which are higher than those of the conventional data reproduction apparatus.
In a CD reproduction apparatus, for example, a disk is used which comprises a transparent plastic disk 12 cm in diameter and 1.2 mm thick, with a thin metal film formed on the transparent plastic disk. Pits (indentation patterns of different reflectivities) are formed in the thin metal film to correspond to digital (PCM) data. The CD disk is scanned by an optical pickup which is moved from the innermost track to the outermost track in the radial direction of the CD disk. An optical pickup which has a semiconductor laser and a photoelectric transducer therein is used to scan the rotating CD disk. The CD disk must be rotated at a constant linear velocity (CLV) all the time. The speed of rotation of the CD disk must therefore continuously change from 500 rpm to 200 rpm as the optical pickup moves from the innermost track to the outermost track. Since the CD disk has a track pitch of 1.6 .mu.m to allow one-hour stereo playing on one side thereof, a great amount of data may be recorded on the CD disk in a program area (at a radius of 25 to 58 mm), and index data thereof are recorded on a lead-in area (at a radius of 23 to 25 mm). As a result, highly precise control functions and the like are required.
In a digital disk player of the type described above, it is very important to drive the disk motor, and hence, the optical disk, at a constant linear velocity, thereby properly reproducing a digital signal from the optical disk.
The disk motor is controlled such that the frequency and phase of a sync signal recorded together with the digital signal coincide with those of a reference signal using a reference clock signal. In this case, pulse-width modulated signals corresponding to deviations in the frequency and phase of the sync signal with those of the reference signal are detected. These pulse-width modulated signals are supplied to a disk motor through a low-pass filter to control the motor rotation at a constant linear velocity.
However, the conventional pulse-width modulating means has many disadvantages and a complex configuration, often resulting in erroneous operation. The conventional pulse-width modulating means is thus unsatisfactory for a variety of practical applications.